Abstract: We constrain the abundance of primordial black holes (PBH) using 2622
microlensing events obtained from 5-years observations of stars in the Galactic
bulge by the Optical Gravitational Lensing Experiment (OGLE). The majority of
microlensing events display a single or at least continuous population that has
a peak around the light curve timescale $t_{\rm E}\simeq 20~{\rm days}$ and a
wide distribution over the range $t_{\rm E}\simeq [1, 300]~{\rm days}$, while
the data also indicates a second population of 6 ultrashort-timescale events in
$t_{\rm E}\simeq [0.1,0.3]~{\rm days}$, which are advocated to be due to
free-floating planets. We confirm that the main population of OGLE events can
be well modeled by microlensing due to brown dwarfs, main sequence stars and
stellar remnants (white dwarfs and neutron stars) in the standard Galactic
bulge and disk models for their spatial and velocity distributions. Using the
dark matter (DM) model for the Milky Way (MW) halo relative to the Galactic
bulge/disk models, we obtain the tightest upper bound on the PBH abundance in
the mass range $M_{\rm PBH}\simeq[10^{-6},10^{-3}]M_\odot$ (Earth-Jupiter mass
range), if we employ null hypothesis that the OGLE data does not contain any
PBH microlensing event. More interestingly, we also show that Earth-mass PBHs
can well reproduce the 6 ultrashort-timescale events, without the need of
free-floating planets, if the mass fraction of PBH to DM is at a per cent
level, which is consistent with other constraints such as the microlensing
search for Andromeda galaxy (M31) and the longer timescale OGLE events. Our
result gives a hint of PBH existence, and can be confirmed or falsified by
microlensing search for stars in M31, because M31 is towards the MW halo
direction and should therefore contain a much less number of free-floating
planets, even if exist, than the direction to the MW center.